Pressure sensor

ABSTRACT

One embodiment relates to a pressure sensor apparatus, including a housing with a flexible member and an aperture configured to receive a fluid. The pressure sensor apparatus further includes a first member disposed on the flexible member, a second member removably coupled to the first member configured to move in response to a pressure of the fluid and a sensor configured to detect the movement of the second member. The pressure sensor apparatus generates a pressure signal for the fluid based on the displacement of the second member

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/658,256, titled “Apparatus with Flexible Member for Sensing FluidPressure,” filed Oct. 23, 2012, which claims the benefit of U.S.Provisional Application No. 61/567,854, filed Dec. 7, 2011, thedisclosures of which are incorporated herein by reference in theirentireties.

BACKGROUND

The present invention relates generally to a pressure sensor for a fluid(e.g., a gas or liquid). A wide variety of mechanisms may be used todetect the pressure of a fluid, including a piezoelectric mechanism, apotentiometer, an electromagnetic mechanism, a capacitor, or apiezoresistive mechanism.

In some applications, a disposable pressure sensor may be utilized. Forexample, a pressure sensor may be utilized in a machine that handlesblood, such as a centrifuge that separates blood into its variouscomponents (e.g., red blood cells, platelets, plasma, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of the present invention will becomeapparent from the following description, appended claims, and theaccompanying exemplary embodiments shown in the drawings, which arebriefly described below.

FIG. 1 is a side view of a pressure sensor in an initial or neutralstate, according to an exemplary embodiment.

FIG. 2 is a side view of the pressure sensor of FIG. 1 in a positivepressure-sensing state, according to an exemplary embodiment.

FIG. 3 is a side view of the pressure sensor of FIG. 1 in a negativepressure-sensing state, according to an exemplary embodiment.

FIG. 4 is a side view of a pressure sensor in an initial or neutralstate, according to an exemplary embodiment.

FIGS. 5A and 5B is a side view of a pressure sensor in an initial orneutral state, according to an exemplary embodiment.

FIG. 6 is an exploded view of a disposable fluid processing assemblyusable in association with a centrifuge assembly.

FIG. 7 is a rear isometric exploded view of a fluid control cassettethat may be incorporated by the fluid processing assembly of FIG. 6.

FIG. 8 is a front isometric view of the fluid control cassette of FIG.7.

FIG. 9 is an isometric view of a cassette holding station for acentrifuge, according to an exemplary embodiment.

FIG. 10 is a flowchart of a method for measuring the pressure of afluid, according to an exemplary embodiment.

FIG. 11 is a side view of a pressure sensor, according to anotheralternative embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

It is to be understood that the following detailed description isexemplary and explanatory only, and is not restrictive of the inventionas claimed.

It is useful to measure the pressure inside a disposable blood-handlingassembly with a pressure sensor in order to determine whether or notthere are occlusions in the flow either upstream or downstream from thepressure sensor. Further, during installation checks, the disposableassembly may be checked for leaks by applying a positive or negativepressure to portions of the disposable assembly. Because portions of theassembly are disposable, and are advantageously inexpensive, some typesof pressure transducers embedded in the disposable assembly would becost prohibitive. Additionally, any sensors in the disposable assemblywould have to withstand sterilization, would have to be biologicallycompatible, and would have to provide safety isolation from thedonor/patient (e.g., be electrically isolated). Therefore, non-invasivepressure measurements are typically performed by either measuring thepressure via a diaphragm in the disposable assembly coupled to a loadcell or via insertion of an open tube, often separated by a sterilityfilter, directly into a pressure transducer.

According to an exemplary embodiment, a sensor apparatus monitors thedisplacement of the diaphragm with a non-contact measurement, forexample, by the sensor being disposed so as not to physically contactthe diaphragm or disposable assembly. The diaphragm may be coupled to aspring with a known spring constant. The spring coupled to the diaphragmallows a near-linear conversion of the linear displacement of thediaphragm to the pressure inside the disposable assembly. The magnitudeof the linear displacement of the diaphragm in the pressure rangebetween a positive pressure position and a negative pressure position isa factor of the spring constant of the spring. The spring constant canbe compensated for in calibration of the sensor apparatus.

Referring to FIGS. 1-3, one exemplary embodiment relates to a sensorapparatus 20 that may be utilized as a part of a larger system. Thesensor apparatus 20 is configured to monitor the pressure of a fluid.Sensor apparatus 20 includes a generally hollow housing 22 with aflexible member 24. Housing 22 and flexible member 24 define an interioror chamber 25 to receive a fluid. The fluid may be contained in a fluidsystem 50 (see FIGS. 6-9). According to an exemplary embodiment, housing22 includes an inlet 26 and an outlet 28 to allow fluid from fluidsystem 50 to enter and exit sensor chamber or pressure region 25. Inother exemplary embodiments, housing 22 may include more than one inlet26 or outlet 28, a single dual inlet/outlet, etc.

According to an exemplary embodiment, flexible member 24 (e.g.,membrane, diaphragm, film, etc.) is formed from an elastomeric or otherflexible material. Flexible member 24 can be a loose membrane that mayor may not be coupled to housing 22, and may or may not be stretchedtight. In an exemplary case in which flexible member 24 is not stretchedtight, the flexible member does not stretch as it is displaced by thepositive or negative pressure of the fluid and therefore the physicalproperties of the membrane can be largely ignored when calculating thepressure. The material chosen, the tightness of the material, and theamount of displacement may be chosen to remove the force required tostretch the membrane from the membrane movement equation. For example,silicone or PVC may be used, though silicone may be preferred in someembodiments because it can stretch farther without plastic deformationand has a lower Young's modulus than PVC.

Flexible member 24 is configured to move in response to a pressure ofthe fluid admitted into chamber 25 through inlet 26. Pressure exerted onflexible member 24 forces flexible member 24 to deform relative to abase or no-load configuration in which an equal pressure is beingexerted on both sides of flexible member 24 (e.g., atmospheric pressureoutside of sensor apparatus 20 and in chamber 25), as shown in FIG. 1.If the pressure in chamber 25 is greater than atmospheric pressure(e.g., a positive pressure in fluid system 50), flexible member 24 isdeformed away from chamber 25, as shown in FIG. 2. If the pressure inchamber 25 is less than atmospheric pressure (e.g., a vacuum or negativepressure in fluid system 50), flexible member 24 is deformed intochamber 25, as shown in FIG. 3. The magnitude of the pressure in fluidsystem 50 (e.g., the pressure differential between chamber 25 andatmospheric pressure) is proportional or otherwise related to therelative deformation or displacement of flexible member 24.

Sensor apparatus 20 further may include a rigid or movable member 30coupled to the outer surface of flexible member 24. According to anexemplary embodiment, rigid member 30 is a disk formed of a ferrousmetal. Rigid member 30 may, for example, be coupled to flexible member24 with a permanent adhesive, vacuum coupling, or other couplingmechanism. In other exemplary embodiments, flexible member 24 may bereplaced with a relatively rigid device, such as a shuttle, rigidcapsule, or a ball bearing that is confined, for example, by a track ortube in fluid communication with chamber 25 and the exterior atmosphere.A rigid movable member may be configured to move back and forth alongthe track or tube in response to a pressure differential between chamber25 and atmospheric pressure. In alternative embodiments, rigid member 30may be molded to, integral with, or otherwise move with flexible member24.

In an alternative embodiment, rigid member 30 may be omitted or replacedwith a second flexible member. In one example, the flexible member 24and/or second flexible member may be embedded with or otherwise compriseiron particles if the coupling is magnetic. Alternatively, in the caseof a vacuum coupling, there could be no member intermediate the flexiblemember and a portion of the remainder of the system. According toanother embodiment, flexible material 24 may comprise ferrous ormagnetic particles such that the distance to sensor 40 can be detecteddirectly without requiring rigid member 30 or magnet 32.

Rigid member 30 provides a mechanism with which a disposable portion ofsensor apparatus 20 that directly interfaces with the fluid (e.g.,housing 22, and flexible member 24) may be coupled to a relativelypermanent portion attached to, for example, a centrifuge systemdescribed below. According to an exemplary embodiment, rigid member 30is coupled to a magnet 32 with a magnetic force. The coupling betweenrigid member 30 and magnet 32 is configured to be temporary, allowingthe disposable portion of sensor apparatus 20 to be easily removed fromthe more permanent portion of sensor apparatus 20. According to otherexemplary embodiments, the magnet may be coupled to rigid member 30 withanother removable coupling mechanism (e.g., an adhesive, a vacuum, aphysical catch, Velcro, a high-friction surface of a material, anelastomeric material, etc.). The coupling of rigid member 30 and magnet32 allows for both positive and negative pressures to be measured viapositive and negative displacement of the interconnected flexible member24, rigid member 30, and magnet 32. The coupling also allows forremovability by force of a person's hand, or without the need for atool.

The movement of magnet 32 is constrained by one or more biasing elementssuch as springs 36 and a magnet housing 38. Housing 38 may includemechanical stops, protrusions, or extensions 37 and 39. As shown in FIG.1, stops 37 and 39 may extend inward from the main body of housing 38and may be configured to limit the travel of magnet 32 as flexiblemember 24 is deformed. Stops 37 and 39 may not directly contact magnet32, but may instead contact an intermediate body, such as a carrier 34.For instance, if magnet 32 is a disk-shaped body, carrier 34 may be agenerally annular body surrounding magnet 32. Stops 37 and 39 provideoverload protection for both excessive positive pressures (stop 39) andnegative pressures (stop 37). Limiting the travel of magnet 32 andcarrier 34 may advantageously prevent magnet 32 or carrier 34 fromcontacting sensor 40 and damaging magnet 32 and/or sensor 40.

Springs 36 bias magnet 32 towards a neutral position, shown in FIG. 1,and provide a force resisting displacement of magnet 32 in either thepositive or negative directions. Springs 36 are coupled on one end tomagnet 32 and on the opposite end to a non-movable body, such as housing38. According to an exemplary embodiment, springs 36 are not coupleddirectly to magnet 32, but instead to carrier 34. The magnitude of thedisplacement of magnet 32 by a known fluid pressure acting on flexiblemember 24 is determined at least partially by the spring constant ofsprings 36. Springs 36 may be configured with a relatively high springconstant such that there is a relatively small displacement of magnet 32between positive fluid pressures as shown in FIG. 2 and negative fluidpressures, as shown in FIG. 3.

The introduction of the spring to balance the rigid member against theforce from the fluid pressure allows the system to use a flexible memberthat provides little resistance to movement—for example, silicone—suchthat the spring force is many times greater than the resistive force ofthe flexible member. Therefore, the spring provides a linear andcontrolled response to the force of the pressurized fluid (F=—kx; wherek is the spring constant and x is the displacement). Thus, in oneembodiment, displacement of the combination flexible member and rigidmember is directly proportional to the pressure of the fluid and onlyvaries based on the spring constant, which is constant over a wide rangeof conditions over time.

The displacement of magnet 32 is detected and monitored by a sensor 40.Sensor 40 transmits a signal to be analyzed by a computer or otherdevice to detect changes in position of magnet 32 (e.g., deflection,translation, etc.) and to calculate or determine the pressure of fluidsystem 50 based on the detected or measured position changes. Sensor 40is positioned such that it is separated from magnet 32 by a gap 42 anddoes not contact magnet 32. Magnet 32 is isolated by springs 36 andrigid member 30 coupled to flexible member 24. Therefore, the forcesacting upon magnet 32 in this exemplary embodiment are the pressureacting on flexible member 24 and the reaction forces of springs 36.Because the displacement of magnet 32 may be small due to smallmagnitude fluid pressures in system 50 or springs 36 with high springconstants, sensor 40 may be a high resolution sensor.

According to an exemplary embodiment, sensor 40 may be a Hall Effectsensor that outputs a voltage that varies with the magnetic field actingon sensor 40. As shown in FIGS. 1-3, sensor 40 is provided at a distancefrom magnet 32 in line with magnet 32. A positive pressure in system 50forces magnet 32 towards sensor 40, reducing gap 42 between magnet 32and sensor 40 and increasing the magnetic field detected by sensor 40. Anegative pressure in system 50 forces magnet 32 away from sensor 40,increasing gap 42 between magnet 32 and sensor 40 and decreasing themagnetic field detected by sensor 40. Magnet 32 therefore provides botha coupling mechanism between the disposable portions of sensor apparatus20 and the non-disposable portions of sensor apparatus 20 and provides ameans for detecting the displacement of flexible member 24. Alternativestructures to magnet 32 may be used to provide either or both of thesefunctions.

Hall Effect sensors can be less expensive than other commonly usedsensors, such as a contact load cell. The resolution of Hall Effectsensors can be very high with a relatively small air gap 42 betweenmagnet 32 and sensor 40. Therefore, very small movements of theinterconnected flexible member 24, rigid member 30, and magnet 32 can bedetected, allowing relatively high resolution conversion of displacementto pressure. In one example, a Honeywell SS490 series MiniatureRatiometric Linear Hall Effect integrated circuit chip may be used. Thesensor may have a ratiometric output voltage, set by a supply voltage,that varies linearly in proportion to the strength of the magneticfield.

Referring to FIG. 11, in another alternative embodiment, a Hall Effectsensor may be positioned to the side rather than below the magnet suchthat the air gap is constant and the magnet moves parallel to thesensor. For a Hall Effect sensor, the change in magnetic field strengthchanges very linearly in proportion to the position in the middleportion (e.g., ˜30%) of the magnet, whereas in the embodiments of FIGS.1-3, the magnetic field strength is logarithmically proportional to theair gap distance.

Referring to FIG. 4, a sensor apparatus 120 is shown according toanother exemplary embodiment. Sensor apparatus 120 is similar to sensorapparatus 20, with an interconnected flexible member 124, rigid member130, and magnet 132 that are displaced by a fluid pressure in a chamber125. However, in this embodiment magnet 132 includes a post 144 thatextends outward opposite of rigid member 130. A magnetic strip 146 withalternating north and south poles is provided along post 144. Thedisplacement of magnet 132 is resisted by a spring 136.

Sensor 140 is located proximate to post 144. Unlike sensor 40, sensor140 is provided along the side of magnet 132 and the gap 142 betweensensor 140 and magnet 132 does not substantially change as magnet 132 isdisplaced by a positive or negative pressure in system 50. Sensor 140detects the displacement of magnet 132 by sensing the alternatingmagnetic fields of strip 146. Sensor 140 converts the switching of N-Spoles caused by the movement of magnet 132 into incremental positionchange data. According to one exemplary embodiment, sensor 140 may be anAS5311 integrated linear Hall encoder, as marketed byaustriamicrosystems AG of Styria, Austria.

Because sensor 140 does not directly sense the magnetic fields of magnet132, but rather senses magnetic strip 146, according to other exemplaryembodiments, the positions of magnet 132 and rigid member 130 may bereversed. Therefore, magnet 132 may be coupled directly to flexiblemember 124 and post 144 with magnetic strip 146 may extend from member130. A magnetic couple is still formed between magnet 132 and member 130and sensor 140 is able to detect the displacement of magnetic strip 146.In other embodiments, there could be no magnet 132 and the coupling ofmagnetic strip 146 to rigid member 130 or directly to flexible member124 could be accomplished in other ways.

Referring to FIG. 5, a sensor apparatus 220 is shown according toanother exemplary embodiment. Sensor apparatus 220 is similar to sensorapparatus 120, with an interconnected flexible member 224, rigid member230, and magnet 232 that are displaced by a fluid pressure in a chamber225. However, post 244 extending from magnet 232 is a threaded memberthat engages a similarly threaded rotary member 248. Rotary member 248is supported by a housing or frame 238. A support or bearing mechanism249, for example ball bearings, may be provided between frame 238 androtary member 248 to support rotational movement. Rotary member 248 ismagnetic and is configured with alternating north and south poles aboutits periphery. A displacement of magnet 232 causes a rotation of rotarymember 248 by the pitch of the threaded connection between post 244 androtary member 248.

Sensor 240 is provided on a side of rotary member 248 and the gap 242between sensor 240 and rotary member 248 does not change as rotarymember 248 is rotated by a positive or negative pressure in system 50.Sensor 240 detects the displacement of magnet 232 by sensing thealternating magnetic fields about the periphery of rotary member 248.Sensor 240 converts the switching of N-S poles into incremental positionchanges. According to one exemplary embodiment, sensor 240 may be anAS5311 integrated linear Hall encoder, as marketed byaustriamicrosystems of Styria, Austria.

Because sensor 240 does not directly sense the magnetic fields of magnet232, according to other exemplary embodiments, the positions of magnet232 and rigid member 230 may be reversed. Therefore, magnet 232 may becoupled directly to flexible member 224 and threaded post 244 may extendfrom member 230. A magnetic couple is still formed between magnet 232and member 230 and sensor 240 is able to detect the displacement ofrotary member 248 as it is rotated by the threaded connection to post244.

As shown in FIG. 1, sensor 40 of sensor apparatus 20 may be coupled to aprocessing circuit 44. Processing circuit 44 is configured to calculatethe pressure of fluid system 50 based on signals received from sensor40. Processing circuit 44 may further be coupled to a display 46 tooutput the calculated pressure readings to a user. A user input device48 may be provided to allow a user to interact with sensor apparatus 20(e.g., by performing diagnostic tests, establishing a reference or zerocondition, changing pressure units, resetting the device, etc.). Sensors140 and 240 may be coupled to similar processing circuits.

Referring now to FIG. 6, according to one exemplary embodiment, fluidsystem 50 may be utilized in a blood processing system to process wholeblood or other suspensions of biological material. To avoidcross-contamination between donors, the blood processing system mayinclude a first portion or housing configured to be reused for multipleblood processing operations for different donors, and a second portionor housing configured for a single blood processing operation. Accordingto one exemplary embodiment, the first portion may be a device such as acentrifuge and the second portion may be a disposable fluid system 50(e.g., a fluid processing assembly). Fluid system 50 is insertable toand removable from the centrifuge. Fluid system 50 includes conduits 52configured to convey fluid through fluid system to and from thecentrifuge or other mechanism. Fluid system 50 further includes one ormore cassettes 54 that direct liquid flow among multiple fluid sourcesand destinations during a fluid processing procedure. A flexiblemembrane or diaphragm 70 is disposed in the cassettes 54. As describedabove, diaphragm 70 is configured to move in response to a change inpressure of the blood in fluid system 50. Fluid system 50 comprises atray 81 which snaps over a cassette holder 80 (see FIG. 9).

As shown in more detail in FIGS. 7 and 8, cassettes 54 have a body orhousing 60 that is formed to include one or more input ports 62, one ormore output ports 64, and channels 66 that direct fluid from inputs 62to outputs 64. A cover or panel 68 is coupled to one side of housing 60while flexible diaphragm 70 is coupled to the opposite side of housing60.

Housing 60 further forms one or more open-ended sensing portions 72.Sensing portions 72 are analogous to chamber 25 of sensor apparatus 20described above. Apertures 61 in housing 60 allow sensing portions 72 tobe in fluid communication with channels 66. The open end of each sensingportion 72 is sealed by diaphragm 70 (e.g., with an ultrasonic weldingoperation, a vacuum, etc.). In this way, diaphragm 70 serves the samefunction as flexible member 24 of sensor apparatus 20 described above.

Referring now to FIG. 9, each cassette 54 is received in a holder 80 inthe centrifuge. Holder 80 may include actuators 82 that interact withaligned valves in cassettes 54 to sense the displacement of diaphragm 70proximate to sensing portions 72. Sensors may be provided adjacent tochambers 84 (e.g., hollows, recesses, cavities, etc.) formed in holder80 that are aligned with sensing portions 72 of cassette 54. Holder 80may therefore be analogous to magnet housing 38 of sensor apparatus 20described above.

Once cassette 54 is inserted into holder 80, the sensor may detect theposition of diaphragm 70 to establish a reference or zero condition. Asa fluid such as blood flows through channels 66 in cassette 54, thedisplacement of portions of diaphragm 70 are monitored. The sensor maythen generate a signal indicative of the movement and transmit thesignal to a processing circuit configured to calculate a pressure of theblood in various channels 66. This data may be used for various otherprocesses associated with the blood processing system, such as operatingvalves in cassettes 54 using actuators.

Referring to FIG. 10, a flowchart of a method 90 for measuring thepressure of a fluid is shown according to an exemplary embodiment. Ahousing with a flexible member is first provided. A rigid member isdisposed on the flexible member (step 92). A second member is thenremovably coupled to the rigid member (step 94). A fluid is channeledinto the housing to the flexible member (step 95). The flexible memberis displaced or deformed in response to a change in pressure of thefluid (step 96). The flexible member may be moved in a first directionin response to an increase in pressure in the fluid and moved in asecond direction opposite the first direction in response to a decreasein pressure of the fluid. The displacement or deformation of the secondmember is detected with a sensor (step 98). The second member may be,for example, magnetic, and the displacement of the second member may bedetected with a magnetic field sensor. A pressure signal may begenerated based on the detected movement of the second member (step 99).

Some embodiments may provide for noninvasive, extracorporeal pressuremonitoring of a fluid, such as blood. Monitored pressure data may beutilized to facilitate the processing of the fluid, such as theseparation of blood into its component parts.

Some embodiments may provide a low cost pressure sensor for systems inwhich some of the system is configured to be disposable. As described,sensor apparatus 20 is configured to include a portion interacting withthe fluid (e.g., housing 22 and flexible member 24) that is relativelylow cost that separates from a second portion that includes moreexpensive components (e.g., sensor 40). The lower cost components maythen be disposed of after each use while the more expensive componentsare retained to be used multiple times.

Some embodiments may provide a pressure sensor with a disposable portionand a non-disposable portion that are coupled together with a temporary,removable coupling mechanism. In this way, the installation of a newdisposable portion may be accomplished quickly and easily withoutnegatively effecting the accuracy of data collected with the pressuresensor.

While the sensor apparatus has been described as utilizing magneticfield sensors to detect the displacement of a magnetic member coupled tothe flexible membrane, in other embodiments, other sensors may beutilized. Such sensors may be non-contact sensors or may be contactsensors. For instance, displacement of the flexible membrane or memberscoupled to the flexible membrane may be detected with optical sensors,eddy current sensors, laser triangulation sensors, LVDT (linear variabledifferential transformer) sensors, potentiometers, etc. In oneembodiment, an optical linear encoder may be used. An optical linearencoder may comprise an optical sensor, transducer or readhead pairedwith a scale that encodes position. The sensor reads the scale in orderto convert the encoded position into an analog or digital signal, whichcan then be decoded into position by a digital controller. In place ofN/S magnets as the scale, in this embodiment a clear or reflective stripof material with surface coatings or etchings creating a Ronchi ordiffraction grating may be used as the scale, such that position can beascertained by optically tracking or reading the movement from one gapto the next. Linear encoders other than optical linear encoders (e.g.,magnetic linear encoders, etc.) may alternatively be used.

The construction and arrangement of the elements of the pressure sensoras shown in the exemplary embodiments are illustrative only. Althoughonly a few embodiments of the present disclosure have been described indetail, those skilled in the art who review this disclosure will readilyappreciate that many modifications are possible (e.g., variations insizes, dimensions, structures, shapes and proportions of the variouselements, values of parameters, mounting arrangements, use of materials,colors, orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter recited. For example,elements shown as integrally formed may be constructed of multiple partsor elements. Some like components have been described in the presentdisclosure using the same reference numerals in different figures (e.g.,housing 22). This should not be construed as an implication that thesecomponents are identical in all embodiments; various modifications maybe made in various different embodiments. It should be noted that theelements and/or assemblies of the enclosure may be constructed from anyof a wide variety of materials that provide sufficient strength ordurability, in any of a wide variety of colors, textures, andcombinations. The processing circuit may comprise any digital and/oranalog circuit components configured to perform the functions recitedherein. The processing circuit may comprise one or more modules, units,circuits, etc., may comprise a microprocessor, microcontroller,application-specific integrated circuit, programmable logic, or othercircuitry. The processing circuit may comprise a tangiblecomputer-readable memory having instructions encoded thereon which whenprocessed by a processor perform the functions recited herein.Additionally, in the subject description, the word “exemplary” is usedto mean serving as an example, instance or illustration. Any embodimentor design described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other embodiments ordesigns. Rather, use of the word exemplary is intended to presentconcepts in a concrete manner. Accordingly, all such modifications areintended to be included within the scope of the present inventions.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions, and arrangement of the preferredand other exemplary embodiments without departing from the spirit of theappended claims.

1. A blood processing system, comprising: a first housing configured tobe reused for multiple blood processing operations for different donors;a second housing configured for a single blood processing operation, thesecond housing insertable to and removable from the first housing, thesecond housing comprising a conduit configured to house blood and apressure sensing portion having a rigid member disposed thereon, therigid member configured to move in response to a change in pressure ofthe blood; and a sensor system coupled to the first housing andconfigured to be reused for multiple blood processing operations, thesensor system comprising a second member and a sensor, the second memberremovably coupled to the rigid member, the sensor configured to detectacross an air gap movement of the second member and to generate a signalindicative of the movement.
 2. The blood processing system of claim 1,further comprising a processing circuit coupled to the sensor systemconfigured to calculate a pressure of the blood based on the signalindicative of the movement, wherein the movement is proportional to thepressure of the blood.
 3. The blood processing system of claim 1,wherein the second housing is part of a cassette comprising an inputport, an output port, and channels including the conduit.
 4. The bloodprocessing system of claim 1, wherein the second housing is disposable.5. The blood processing system of claim 1, further comprising a springcoupled to the first housing configured to bias the second memberagainst the rigid member.
 6. The blood processing system of claim 5,further comprising a plurality of stops protruding from a portion of thefirst housing configured to limit the movement of one of the rigidmember and second member.
 7. The blood processing system of claim 1,wherein the sensor system comprises a linear encoder comprising a sensorand a scale coupled to or disposed on the rigid member, the sensorconfigured to detect scale changes due to movement of the rigid member.8. The blood processing system of claim 2, wherein the processingcircuit is coupled to a display to output the calculated pressure to auser.
 9. The blood processing system of claim 2, further comprising auser input device configured to allow a user to interact with the sensorsystem to perform at least one of performing a diagnostic test,establishing a reference or zero condition, change a pressure unit, orreset the sensor system.
 10. The blood processing system of claim 2,wherein the second portion includes valves, and the signal indicative ofthe movement is used to operate the valves using actuators.
 11. Theblood processing system of claim 1, wherein the first housing is acentrifuge.
 12. The blood processing system of claim 1, wherein thesecond member is removably coupled to the first member by at least oneof an adhesive or vacuum coupling.
 13. The blood processing system ofclaim 3, wherein the sensor system is in fluid communication with thechannels of the cassette.
 14. The blood processing system of claim 1,wherein the sensor system is configured to measure both positivepressure and negative pressure.
 15. The blood processing system of claim6, wherein the plurality of stops provide overload protection for bothexcessive positive pressures and negative pressures.
 16. The bloodprocessing system of claim 1, wherein the sensor is at least one of anoptical sensor, an eddy current sensor, a laser triangulation sensor, ora linear variable differential transformer sensor.
 17. The bloodprocessing system of claim 1, wherein the second member is magneticallycoupled to the rigid member.
 18. The blood processing system of claim17, wherein the second member is configured to be moved in a firstdirection in response to an increase in pressure of the blood and in asecond direction opposite the first direction in response to a decreasein pressure of the blood.
 19. The blood processing system of claim 18,wherein the sensor is configured to detect movement of the second memberbased on sensing a change in a magnetic field produced by the secondmember.
 20. The blood processing system of claim 1, wherein the sensoris a Hall Effect sensor that outputs a voltage that varies with amagnetic field acting on the sensor.